The mammalian epidermis serves many functions, amongst which are formation and maintenance of a cohesive permeability barrier that guards against excessive transcutaneous water loss and as an external barrier against microbial attack. The outermost layer of the epidermis, the stratum corneum, has a strongly acidic pH, with the pH of the upper stratum corneum being approximately 4.5-5.0 and the pH of the lower stratum corneum approaching neutral. Thus, the stratum corneum experiences a pH differential of more that two pH units over a vertical space of less than 100 microns; a dramatic biological phenomenon. This pH gradient occurs not only in human skin, but also in rodent skin despite its much thinner stratum corneum.
Although first recognized decades ago, an understanding of the origin and function of the buffering capacity of the stratum corneum, often referred to as the “acid mantle”, is incomplete. Whatever the nature of its origin, however, it is known that the acid mantle is essential to normal functioning of the stratum corneum. For example, the acidic pH of the human skin assists in both the regulation of the cohesion of the stratum corneum as well as its barrier function.
The acidic pH of the acid mantle also appears to be necessary for maintaining the lipid barrier, which makes the skin impenetrable to bacteria. The pH of healthy skin naturally increases and recovers, but recovery slows as people age. At higher pHs, around neutral or basic, the lipid barrier deteriorates and the repair process is inhibited. For example, the hydrolase enzyme, Beta-glucocerebrosidase, assists in the post secretory processing of polar lipid precursors into nonpolar lipid products within the stratum corneum interstices. The optimum pH of Beta-glucocerebrosidase is approximately 5.5; however, its activity is inhibited at a lower pH of 3.5-4, and is absent or significantly reduced at a neutral pH. Thus, in developing skin care products a particularly desirable effect would be to both reduce the stratum corneum pH as fast as possible to a pH of about 5.5 (i.e., the optimum pH of Beta-glucocerebrosidase) and maintain the pH at that level.
Most skin care products are neutral or slightly alkaline and are relatively stable toward pH changes after topical application. Unfortunately, a neutral or alkaline pH is not favorable for repair of the stratum corneum lipid barrier. Acidic skin care products with a pH matching that of the acid mantle have also been used for treating the skin. Typically, these products use ordinary acids, such as citric acid, hydrochloric acid or retinoic acid, at concentrations of less than 0.1 weight percent which reduces the pH of the product but does not provide buffering effect to lower the pH of the acid mantle. If added at higher concentrations, the ordinary acids can harm the skin, are highly irritating, and often result in peeling of the skin. Alternatively, low ester pectin has been used as an acidic material for providing buffering effect without harm to the skin. Since low ester pectin being a polymeric material has significantly higher molecular weight than ordinary acids, pectin does not penetrate and harm the skin as does the ordinary acids. However, pectin presents some difficulties in use with respect to its rheological properties and its limiting ability to provide buffering effect due to low ester pectin having a pH of about 4.
The rheological properties of the compositions, which affect the visual appeal and desired flowability and spreadability during use, are an equally important feature in skin care products. For example, thick, non-tacky and non-astringent skin care formulations are often made with polyacrylate, which have a molecular weight on the order of 100,000 and provide the desired rheological properties. When made with natural polysaccharides (e.g., carrageenan or xanthan gum), however, the skin care formulations become tacky and astringent. Without being bound of theory, it is believed that the molecular weight of many natural polysaccharides (e.g., carrageenan or xanthan gum) is at least in part responsible for the tackiness and the astringency that are observed—i.e., carrageenan and xanthan gum have molecular weights on the order of 1 million and 10 million, respectively, as compared to the significantly lower molecular weight of polyacrylates.
Although pectin has a molecular weight comparable to polyacrylates and can provide the desired buffering effect, pectin without the use of other rheological imparting ingredients lacks any texture and is liquid. Prior art compositions including pectin generally contain 1.5 weight percent or less of pectin and rely on the use of other ingredients to impart the desired texture and rheology to the compositions. Commonly used texture-modifying ingredients include emulsifiers; however, the emulsifiers also will emulsify the skin fat. Low ester pectin having a degree of esterification in the range of 5 to 10 has been used in skin care formulations for its buffering effects. However, such pectin products are unable to provide texture to the preparation, which means that other ingredients are necessary to provide the desired texture.
Thus, there remains a need to develop a personal care composition that has desirable textural and rheological properties without requiring use of polyacrylates, emulsifiers, and low molecular weight acids that may irritate the skin.